1. Field of the Invention
The present invention relates generally to the detection of toxic species and, in particular, to a method and apparatus for detecting polar chemical agents and toxic species using conductive polymer coated sensing electrodes which exhibit renewable sensing characteristics enabling many repeated uses.
2. Description of the Prior Art
Electrochemical reactions based on oxidation or reduction (Redox) of metals and compounds at an electrode are highly selective because of the characteristic Redox potential at which the oxidation or reduction of the electroactive species occurs. With electrochemical sensing, selection of the electrode material and electrolyte solution is very important in determining sensitivity and selectivity. Theoretical considerations for Redox electrochemistry are developed in more detail below.
An electrochemical cell utilized for the detection of toxic species normally consists of a working or sensing electrode, counter electrode and a reference electrode situated such that an adjustable potential source may be connected across the sensing and counter electrodes and the current measured. This system includes an electrolyte solution containing a solvent and an electrolyte material together with a depolarizer, if desired. The solvent and electrolyte solutions from such solvents should be electrochemically stable to oxidation and reduction in the operating range of the electrochemical cell. This is necessary to give the widest possible voltage window to carry out electrochemical Redox reactions at the electrode surface without causing the decomposition of the solvent. Thus, the oxidation and/or reduction potentials of the sensing electrode must be lower than the decomposition potential of the electrolyte solvent.
One such cell which utilizes a non-aqueous, aprotic electrolyte system and includes a platinum sensing electrode is illustrated and described in U.S. Pat. No. 4,555,690 to H. V. Venkatasetty, the inventor of the present invention. That application is assigned to the same assignee as the present application.
Conducting polymeric materials are also known in the prior art and certain conducting polymers have also been utilized in fabricating polymer-coated platinum wire electrodes. Properties of certain polyquinoline polymers and substituted polyquinoline polymers as conductors together with their use as coatings on platinum electrodes are discussed by A. H. Schroeder, et al and Y. S. Papir, et al, respectively in abstracts numbers 543 and 544 of the Extended Abstracts of the Electrochemical Society, Spring Meeting, 1983.
Although prior art sensors have been quite successful in detecting chemical agents and toxic chemicals of interest, electrochemical cells of the type described in the above-referenced U.S. Patent use platinum sensing electrodes whereas in the present invention a variety of sensing electrodes are available. Thus, one may choose the most sensitive one for detecting a particular chemical of interest. Each polymer coated electrode has its own characteristic voltammogram related to the particular polymer used. The pure platinum electrode by itself, however, does not have well defined voltammogram.